Mexican Gasoline

[gn7]

My guess is pemex may have a higher sulfur count and less detergents in it.

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My guess is it is not what they putting in, but what they aren't removing, and what the actual chemistry is in their blend.
Gasoline isn't just one compound, its dozens. The gasoline in the US, and particularly Cal and NY, and some other major cities in the country is a completely different formulation than other parts. But all gas in the US is what they call "reformulated gasoline". If you sold this stuff in 60s and 70s, you would go to jail because it doesn't come near the definition of gasoline as described by the feds back then.

When cat converters first arrived, the smell from cars was disgusting. When the sulfer in the fuel hits the converter, along with any unburnt hydrocarbons, the heat in the convertor makes it form hydrogen sulfide. Rotten eggs.

My guess is that Premex doesn't go thru the process to eliminate the sulfer in the fuel and that's what you are smelling.